The present invention relates to a welding method for welding a quartz glass tube for use as an optical fiber preform with a dummy tube; in further detail, it relates to a welding method for welding a large quartz glass tube for use as an optical fiber preform with a dummy tube.
Recently, a larger amount of optical fibers, particularly, the single mode optical fibers, are being used with increasing practical use of optical fibers. However, it is expected that a still larger amount of optical fibers will be necessary with the expansion in its field of usage ranging from long distance main communication lines to domestic lines. To meet with such a demand in the expansion of usage, it is indispensable to realize mass production and cost reduction in the production of optical fibers. This can be accomplished most simply by forming a large optical fiber preform and by then drawing it. In the conventional practical method for producing optical fibers such as the vapor-phase axial deposition process (VAD process) or the outer vapor-phase deposition process (OVD process), the core portion and the clad portion are all produced by VAD or OVD process. Thus, in case of scaling up, there was a disadvantage that the productivity of the fiber optical preform may be lowered. Furthermore, if it is tried to form a larger porous body before vitrification into a transparent body (that is, a soot body obtained by depositing fine silica glass particles, and is referred to hereinafter as ,a porous soot body), there may generate troubles such as the generation of cracks or the drop off of the porous soot body, resulting in great reduction of the productivity. As a method for producing an optical fiber which overcomes the above problems, in Japanese Patent Laid-Open No. 109141/1995 and the like is proposed a production method, i.e., the so-called rod-in-tube method, which comprises forming the quartz glass tube for use as the clad portion accounting for 80% or more of the cross section area by a method capable of producing a high performance quartz glass tube and yet of reducing cost, and then monolithically integrating the resulting quartz glass tube with the core glass rod formed by, for example, the VAD process or the OVD method.
In the production method described in the above Laid-Open Japanese patent application, in order to lower the production cost of the optical fiber, for instance, as shown in the Example described therein, a low cost dummy tube is welded in the front end of the expensive quartz glass tube for use in the optical fiber preform, and transportation, melt welding into a monolithic body, etc., are conducted by clamping the dummy tube. This welding method comprises heating and melting both tubes and then directly pressing the edges thereof against each other without using a welding rod, and, after welding, if necessary, the outer surface was shaped by pressing the outer circumferential surface by use of a graphite trowel and the like. In case of using such a welding method, the shortcomings were ignored even if the welding before pressing was insufficient, and hence, there was a danger of causing accidents such as the falling off of the welded portion.
Under such circumstances, it has been thought of sufficiently melting the welding edges of the quartz glass tube for use as the preform and the dummy tube, and then pressing against each other by a sufficiently strong force. However, in case where it was sufficiently melted and pressed against each other with a sufficiently strong force, there occurred deformation at the welded portion, such that the quartz glass tube for use as the preform and the dummy were mutually intruded into each other, or the edges were rounded, thereby leading to the contraction of the inner diameter while expanding the outer diameter. The expansion of the outer diameter is not problematic unless the degree is extreme, but when the contraction of the inner diameter occurs, the scheduled difference between the outer diameter of the core glass rod for use as the preform and the inner diameter of the quartz glass tube for the preform (that is, the clearance between the core glass rod and the quartz glass tube, which hereinafter is referred to as xe2x80x9cclearancexe2x80x9d) causes difficulty in inserting the core glass rod for use as the preform, and forms flaws on the inner circumferential surface of the quartz glass tube for use as the preform. As a result, bubbles were included in the welded interface of the preforms for use as the optical fiber, or, at worst, it was sometimes impossible to insert the core glass rod for use of the preform, and this made the production of the preforms for use as the optical fiber unfeasible. It is thinkable to use a quartz glass tube for use as the preform having a sufficiently large inner diameter which matches with the clearance taking the contraction of the inner diameter into account However, taking such a large clearance makes it difficult to realize a uniform monolithic body upon melt welding together, thereby leading to disadvantages such as a large eccentricity of the preform for use as the optical fiber obtained as the product, or an increase in the probability of increasing the elliptical degree of the preform and/or the core.
Accordingly, the present inventors have extensively continued studies, and, as a result, found that a welding resulting in high welding strength and free from contraction in inner diameter can be realized by chamfering the inner edge portion of the dummy tube and/or the quartz glass tube for use as the optical fiber preform before welding the quartz glass tube for use as the preform with the dummy tube, and then heat melting them to join them together. The present invention has been accomplished based on these findings.
An object of the present invention is to provide a welding method which results in a high welding strength and which does not cause contraction of the inner diameter of the welded portion.
To accomplish the above object, the present invention relates to a welding method wherein a dummy tube is welded to a quartz glass tube for use as an optical fiber preform, characterized by comprising chamfering the inner edge portion of the dummy tube and/or the quartz glass tube for use as the optical fiber preform before welding said quartz glass tube for use as the preform with the dummy tube, and then heat melting them to melt welding them together.
The aforementioned quartz glass tube for use as the preform can be produced by a method comprising, for instance, forming a porous soot body (the porous soot body formed from fine particles of silica glass is hereinafter referred to as xe2x80x9ca porous soot bodyxe2x80x9d) by depositing fine silica glass particles generated by flame hydrolysis of a high-purity silicon tetrachloride or a siloxane compound such as an organic silicon compound in an oxyhydrogen flame, and obtaining a quartz glass tube by subjecting to mechanical grinding, the quartz glass ingot obtained by vitrifying the thus obtained porous soot body into a transparent body, or the quartz glass ingot obtained by subjecting a quartz powder prepared by crushing a naturally occurring quartz to chemical treatment to obtain a purified quartz powder, and then subjecting the resulting powder to Verneuil""s process using an oxyhydrogen flame. Because a quartz glass tube for use as the preform is expensive, it should be effectively used, and, in general, a dummy tube is connected in case of inserting the core glass rod for optical fiber preforms into the quartz glass tube for use as the preform and heating them together to obtain a melt welded monolithic body. As the dummy tube, used are the lower quality cheaper quartz glass tubes such as those containing a larger amount of impurities, bubbles, etc., as. compared with the above described quartz glass tube, and these dummy tubes are thinner in thickness and have an inner diameter equal to or larger than the quartz glass tube for use as the preform. The above described dummy tubes are provided to both ends of the quartz glass tube for use as the preform, and the supporting side of the dummy tube is called as the handling tube, end dummy, etc., whereas that on the opposite side thereof is called as the start dummy, etc. In the present invention, these tubes are collectively denoted as xe2x80x9cdummy tubesxe2x80x9d. In joining the dummy tube with the quartz glass tube for use as the preform, no welding rod is used, and the edge portions of the dummy tube and/or the quartz glass tube for use as the preforms are heated and molten by using an oxygen/hydrogen burner, a propane/oxygen burner, or an electric furnace to melt weld them together. In the welding method according to the present invention, prior to the welding of the dummy tube and the quartz tube, the inner edge portion on the welding surface side of the dummy tube and/or the quartz glass tube for use as the optical fiber is chamfered, and then fused for melt welding. In this case, preferably, the quartz glass tube for use as the preform and the dummy tube are both chamfered. In particular, when the inner diameter of both tubes is equivalent to or close the inner diameter of the quartz glass tube for use as the preform, it is necessary to chamfer the inner edge portions of both tubes, as shown in FIG. 1, but in case the difference between the inner diameter of the tubes is large, chamfering of either one is sufficient. In this case, as is shown in FIG. 2, preferably, the inner edge portion of the tube having a smaller inner diameter is chamfered. More specifically, in case where the difference in inner diameter of the tubes is 20% or more of the wall thickness of the tube having the smaller inner diameter, it is preferred to chamfer the edge portion of only the tube having the smaller inner diameter, because the operation load attributed to processing and the like can be reduced. By thus chamfering, no contraction of inner diameter results even if there occurs an expansion attributed to the pressing applied during welding. Although depending on the inner diameter, the wall thickness, etc., of the quartz glass tube for use as the preform and the dummy tube, the amount to be chamfered is preferably 2 mm or more but not more than 30% of the wall thickness. If the chamfering should be performed at an amount less than the aforementioned range, there is no effect of chamfering but leads to the contraction of the inner diameter. If the amount of chamfering should exceed the range above, the wall thickness of the chamfered portion becomes too small as to lower the strength. For the chamfering of the edge portions of the glass tube and the like, a linear chamfering known as the so-called xe2x80x9cC-type chamferingxe2x80x9d is mostly employed; in the present invention, although a similar effect can be obtained by a non-linear chamfering known as the so-called xe2x80x9cR-type chamferingxe2x80x9d, it is preferred to employ the C-type chamfering from the viewpoint of reducing the operational load such as processing.
Preferred embodiments according to the present invention are described below by way of examples, but it should be understood that the present invention is by no means limited thereto.